The invention is based on a method for actuating at least one electromagnetic valve containing at least one coil which can be excited by way of an excitation current, and an armature, wherein the excitation current can be set in such a manner that, in response to a signal for actuating the armature from one position into another position, it causes the armature to move as a result of an excitation current threshold being exceeded, and is kept at a value which is lower by comparison in order to hold the armature in a specific position.
A method according to the type in question is described, for example, in DE 197 42 038 A1 or in WO 94/19810 A.
Electromagnetic valves of this type may be switching valves or proportional valves. Electromagnetic switching valves, such as, for example, 2/2-way directional control valves or 3/2-way directional control valves, as used in antilock braking systems (ABS) or electronic braking systems (EBS) of vehicles, generally have an electromagnetic coil which can be excited by an exciting current. In order to switch said valves over from one switching position into another switching position in response to an actuating signal or a switching request which is controlled by an electronic control device, the exciting current has to exceed a certain exciting current threshold in order to be able to cause a corresponding movement of the armature. However, no switching over takes place at an exciting current below said exciting current threshold. In order then to hold the armature in its switching position counter to the action of a resetting spring, an exciting current lower than the exciting current switching threshold suffices.
In general, the current flowing through the coil of an electromagnetic valve of this type depends on the ambient temperature. Since, at low temperatures, the ohmic resistance of the coil is lower than at higher temperatures, the current flowing through the coil at low temperatures is greater than at higher temperatures.
Control devices which are formed from a plurality of such electromagnetic switching valves, in particular control devices of antilock braking systems (ABS), are known from the prior art. If a control device of this type is installed in a vehicle and the vehicle, for example when used in polar regions, is exposed to very low temperatures, on account of the ohmic resistance of the coils, which is very low, relatively high currents flow given a predetermined voltage, in particular if all of the solenoid valves of the control device are excited simultaneously. For example, 3.3 amps flow through the coil of an ABS solenoid valve at an ambient temperature of −40° C. These high currents may damage the electrical wiring of the control device and/or blow an electrical fuse.
EP 0 933 274 A2 discloses a hydraulic braking system with solenoid valves which control the flow of hydraulic fluid to and from brake actuators which are actuated by pressure medium. The hydraulic fluid is heated by means of the solenoid valves in order to reduce the viscosity of the fluid.
DE 100 59 348 A1 describes a method for actuating a braking device, in which valve coils necessary for hydraulic control functions are actuated for the purpose of electrically heating one or more supports holding said coils.
By contrast, the present invention is based on the object of further developing a method of the type mentioned at the beginning in such a manner that, even at very low temperature, the currents flowing through the coil of an electromagnetic valve are limited.
The invention is based on the concept of supplying an electromagnetic valve with an exciting current which is greater than zero but is below the exciting current threshold even if no request for actuating or for holding the armature is present. This is because the energizing of the coil generates heat in its turns, as a result of which its ohmic resistance rises, which in turn results in lower currents through the coil. The purpose of energizing the coil according to the invention consequently is to reduce the current through the coil for future requests for actuating or holding the armature.
If a plurality of such valves are combined in one constructional unit, for example in a control device, an advantageously low current loading of the electrical wiring arises even at very low ambient temperatures. In addition, the probability that a fuse will be blown is reduced. Owing to the fact that the exciting current supplied only for heating purposes is always below the exciting current threshold, an inadvertent switching over of the switching valve is avoided.
The exciting current is applied to the coil at a certain time interval after a holding phase of the armature is ended. It is then ensured that the armature is returned again into its initial switching position, and the holding phase is not affected by the heating current.
An exciting current below the exciting current threshold is particularly preferably applied to the coil as a function of the ambient temperature. Particularly if the ambient temperature drops below a lower threshold temperature, the coil is heated by the exciting current, but without causing a movement of the armature.
An actuation for heating purposes before a first functionally induced actuation is also highly advantageous if the temperature of the valve is equal to the ambient temperature. The energizing of the valve for heating purposes can be triggered, for example, by the ignition in a motor vehicle being switched on.
An exemplary embodiment of the invention is illustrated below in the drawing and explained in more detail in the description below.